Electrically conducting organic polymers have been of scientific and technological interest since the late 1970's. These relatively new materials exhibit the electronic and magnetic properties characteristic of metals while retaining the physical and mechanical properties associated with conventional organic polymers. Technological application of these polymers are beginning to emerge.
Today, conductive polymers and composites such as mentioned above have a broad range of applications including their use as materials for carriers of electrically sensitive devices which prevent electrostatic charge (ESC) which may attract airborne particles on critical surfaces and electrostatic discharge (ESD) which may cause device malfunction.
In addition, conducting polymers can be used as machine covers for electronic equipment which prevent the ingress or egress of electromagnetic signals in order to meet the guidelines established by the FCC as to the accepted levels of unwanted electrical noise.
The materials currently in use are rendered conductive through the use of conductive fillers like metal, carbon particles or chemicals such as ionic salts. The problems associated with these materials include high cost, sloughing of the filler, dependency on environmental conditions, and a very high surface resistance.
Polyanilines are known to be a class of soluble, processable electrically conducting organic polymers. This family of polymers displays a range of solubilities in organic and aqueous acid solutions.
Polyanilines are rendered conducting by treatment with cationic reagents (Lewis acids), most commonly protonic acids. Also the polyaniline can be doped by taking the non-conducting form of the polymer and amine triflate salts (which thermally generate acid) and mildly heating them together in the form of a film or in solution. An example of this method is disclosed in U.S. Pat. No. 5,198,153. Although polyaniline is very inexpensive to produce, some of its physical properties such as the impact strength, tensile strength, etc., may limit the full scope of its uses.
There is specific prior art that discloses blending polyaniline with a dopant. U.S. Pat. No. 4,851,487 discloses the doping reaction of polyaniline with anhydrides and the uses of polyimide oligomers having anhydride terminated functionality (R--CO--O--CO--) as dopants.
U.S. Pat. No. 4,855,361 discloses an anhydride doped polyaniline blended with polyimides to form a non-compatible polymeric composite.
The techniques disclosed in these references are completely different from the present invention. The present invention uses polydopants, for example, polyimide precursors such as the polyamic acid (--COOH) form (with a high molecular weight as made) as direct dopants for the polyaniline to obtain conducting blends of the two polymers in one step. In the case of polyamic acid, the polyaniline becomes protonated by the polyamic acid. In the prior art, an anhydride reacted polyaniline is blended with polyimides.
By contrast, in the present invention, the conductive blend is obtained in a single step due to the interaction between the polydopant (polyamic acid) and the conducting polymer leading to a compatible conducting polymer blend. The resultant blend in the present invention has dispersion at a molecular scale as opposed to the prior art wherein the dispersion is at a much coarser scale.
The references cited above do not form a conducting complex or blend with the polyamic acid but rather the polyaniline is reacted with anhydrides first to obtain a product, and thereafter, this product is blended with another polyimide. There is no polyamic acid doping disclosed in the references.
The present invention uses polydopants, which as noted above for the purpose of the present invention, are Lewis acid polymers. Examples of such polydopants are: polyacrylic acid, polysulfonic acid, cellulose sulfonic acid, polyamic acid, photosensitive polyamic acid, polyphosphoric acid, acid chloride (--COCl) containing polymers and sulfonyl chloride (--SO.sub.2 Cl) containing polymers.
This is exemplary and should not be construed as limiting the scope of the polydopants.
The advantages of the use of such materials are that no external corrosive monomeric or oligomeric dopants are necessary; there is high thermal and electrical stability due to the polymeric counteranion; and there is enhanced processability.
It is important to note that because of the interaction of the two polymers as stated above, compatible molecularly mixed blends are formed wherein there is no phase separation. Finally, the solution gels over time which allows the formation of highdraw ratio fibers.